Various electronic devices are mounted on a motor vehicle as a mobile unit. Therefore, the motor vehicle includes a wire harness for transmitting power from a power source and control signals from a computer to the electronic devices. The wire harness includes a plurality of electric wires and connectors attached to an end of the wires.
The wire includes an electrically conductive core wire and a coating made of insulating synthetic resin, which coats the core wire. The wire is a so-called coated wire. A connector includes a terminal fitting and a connector housing that receives the terminal fitting therein. The terminal fitting, consisting of electrically conductive sheet metal or the like, is attached to an end of the wire and electrically connected to the core wire of the wire. The connector housing made of electrically insulating synthetic resin is formed in a box-shape. When the connector housing is connected to the electronic devices, each wires is connected to the corresponding electronic device through the terminal fitting, thereby the wire harness transmits the desired electric power and signals to the electronic devices.
When the wire harness is assembled, first the wire is cut into a specific length and then the terminal fitting is attached to an end of the wire after removing the coating near the end. A wire is connected to another wire according to the need. Afterward, the terminal fitting is inserted into the connector housing, thereby assembling the wire harness.
The wire of the wire harness must be distinguished in terms of the size of the core wire, the material of the coating (concerning with alteration in the materials depending upon heat-resisting property), and a purpose of use. The purpose of use means, for example, an air bag, antilock brake system (ABS), control signal such as speed data, and system in a motor vehicle in which the wire is used, such as a power transmission system.
The coating of the wire used in the wire harness has been colored to a desired color by mixing a coloring material of the desired color with synthetic resin which constitutes the coating when the synthetic resin of the coating is applied onto the circumference of the core wire by extrusion (for example, see Patent Documents 1 to 3). In this case, when a color of an outer surface of the wire is altered, it is necessary to halt an operation of an extrusion apparatus that performs the extrusion-coating. That is, whenever the color of the wire is changed, it is necessary to halt an operation of an extrusion apparatus, causing increasing in a time period and labor hour required for the production of the wire and deteriorating in the productivity of the wire.
Alternatively, the coloring material to be mixed has been replaced when the extrusion apparatus is performing the extrusion-coating. In this case, right after changing the color of the coloring material, a wire, in the color of the synthetic resin of which a coloring material before the replacement and a coloring material after the replacement are mixed, has been inevitably manufactured, causing the deterioration in the yield of the material of the wire.
In order to prevent the deterioration in the productivity of the wire and in the yield of the material of the wire, the present applicant proposed a method, in which monochromatic wire is produced, then the outer surface of the wire is colored with a desired color according to the need, thereby assembling a wire harness (see Patent Document 4). Alternatively, the present applicant proposed an apparatus for coloring wire, by which upon coloring a monochromatic wire, a liquid coloring material is spouted onto the outer surface of the wire with a specific amount thereof at a time so as to allow the spouted coloring material to adhere to the outer surface of the wire, thereby coloring the wire with the desired color (see Patent Document 5).
The coloring apparatus described above includes a coloring unit that spouts the liquid coloring material onto the outer surface of the wire. The coloring unit includes an electromagnetic valve (solenoid valve) that spouts the coloring material onto the outer surface of the wire when the valve is open, and stops spouting the coloring material onto the outer surface of the wire when the valve is closed. The electromagnetic valve is to be opened or closed according to whether voltage is applied to the solenoid or not.
For opening the electromagnetic valve of the apparatus for coloring wire described in the Patent Document 5, a relatively high voltage (hereafter referred to as “spike voltage” in this description) is first applied for a predetermined period. Then, for keeping the electromagnetic valve open, relatively low voltage (hereinafter referred to as “hold voltage” in this description, of course the hold voltage is lower than the spike voltage) is applied for a predetermined period. Thus, the coloring unit spouts the liquid coloring material onto the outer surface of the wire by opening the electromagnetic valve. Then, after drying a droplet of the coloring material deposited on the outer surface of the wire, the outer surface of the wire is colored in a desired color, and marked with a desired mark.    [Patent Document 1] JP-A, H05-111947    [Patent Document 2] JP-A, H06-119833    [Patent Document 3] JP-A, H09-92056    [Patent Document 4] WO03019580    [Patent Document 5] Japanese Patent Application No. 2003-193904
Preferably, the apparatus for coloring wire described above is mounted on various processing apparatuses for processing electric wire, for example, a wire cutting apparatus that cuts the wire to a predetermined length, and mounts a terminal fitting on an end of the cut electric wire. The processing apparatus for electric wire transfers the electric wire in a longitudinal direction, or stops transferring the wire for processing the wire in various ways, for example, cutting the wire or mounting the terminal fitting described above. Accordingly, a transfer speed of the wire is changed rapidly.
On the other hand, for coloring the outer surface of the wire in a specific pattern even if the transfer speed of the wire is changed, the apparatus for coloring wire includes, for example, an encoder for detecting the transfer speed or transferred length of the wire. Further, the apparatus for coloring wire changes time intervals (frequency) of opening the electromagnetic valve according to the transfer speed or transferred length of the wire measured by the encoder for coloring the outer surface of the wire in a specific pattern.
However, sometimes the transfer speed of the wire moved by the processing apparatus is so increased to make a frequency of applying the voltage higher than an upper-limit frequency in which the electromagnetic valve is able to be opened and closed. In this case, the electromagnetic valve keeps opening when the frequency is higher than the upper-limit frequency. Further, the spike voltage is applied to the solenoid of the electromagnetic valve a plurality of times when the frequency is higher than the upper-limit frequency.
In this case, since the electromagnetic valve keeps opening when the spike voltage is applied to the solenoid a plurality of times, the solenoid may be heated. Resultingly, the repeatedly heated solenoid may reduce a lifetime of the electromagnetic valve.
Accordingly, an object of this invention is to provide a driving method of an electromagnetic valve, a driving unit for driving the electromagnetic valve, and an apparatus for coloring electric wire to allow a lifetime of the electromagnetic valve to increase.